1. Field of the Invention
The present invention relates generally to a servo control system for a disc record reproducing apparatus, and more particularly to a servo control system for an apparatus suitable to optically reproduce a PCM (pulse code modulation) signal recorded on a disc record.
2. Description of the Prior Art
In the art, when a PCM signal is recorded on a disc record, there are two methods one of which uses a constant angular velocity of the disc record and the other uses a constant line velocity of the disc record. In the case of the constant line velocity recording, its recording density is greatly improved as compared with that in the case of the constant angular velocity recording, but in the case of the constant line velocity recording it is necessary to reproduce the disc record with its line velocity being constant, which makes it difficult to control the rotation of the disc record during reproduction.
In the case of a video disc record, the rotation control thereof is carried out in such a manner that a reproduced signal is demodulated to provide a composite video signal, the synchronizing signal is then separated from the composite video signal, and the frequency of the synchronizing signal is maintained constant.
In the case of a PCM audio signal wherein the base band is recorded, which is generally a binary signal, the above method can not be employed. If the PCM audio signal is not recorded as the base band recording, but is recorded after it is converted into a quase or pesudo video signal containing a sychronizing signal, the method which is same as that used in the case of the video disc record can be employed. However, in this case the recording density is lowered which will cancel the advantages of the constant line velocity recording.
On the other, such a method may be considered that the position of a pick up device in the radius direction of a disc record is detected by using a mechanical position detecting means, the rotation of the disc record is controlled by the detected output, a clock component is extracted from a reproduced signal under the above conditions, and the extracted component is used as a comparing signal to further control the rotation of the disc record. However, the employment of such mechanical position detecting means results in greatly increased cost of the apparatus.